Combinations of active muscle units are primarily responsible for the three-dimensional for the three-dimensional output of the musculoskeletal system. Because of its ability to sample a whole region of a limb, we will use a novel approach, dynamic magnetic resonance (MR) imaging, to determine where (spatial arrangement) active muscle units are located. The experimental aims of this project will allow us to test the following main hypotheses. First, dynamic MR times are correlated with EMG activity, and thus form a valid assay system for the participation of muscular elements in motor tasks. Second, the spatial arrangement of active whole muscles and/or subvolumes of whole muscles are affected by the requirements of a task. The tasks chosen are sagittal and non- sagittal movements of the ankle under concentric and eccentric conditions. Third, the spatial arrangement of active whole muscles and/of subvolumes will be altered following neuromuscular dysfunction produced by peripheral nerve problems. We predict that peripheral nerve injury will produce a change in the spatial pattern of muscle unit activity induced by one or more of the exercise tasks. Finally, in collaboration with Project 4, we will use turning while making as another independent while walking as another independent measure of function. Turning has been chosen because many patient subjects due well with straight line walking, but then have difficulty when they are forced to turn or step around an object.